Prostate cancer is the most prevalent cancer among American men and is classified as the second leading cause of their cancer mortality. In the United States, there will be 220,900 new cancer cases in 2003 making prostate cancer one of the cancers with the fastest rising incidence in this country as well as in Western Europe. While certain dietary, genetic, lifestyle and environmental factors are implicated in prostate cancer risk, the molecular mechanisms underlying the etiology of the disease are largely unknown. Mutagenic oxidative DNA base damage increases with age in prostatic tissue. Many factors may influence this increase including: increased production of reactive oxygen species, increased susceptibility to oxidative stress, alterations in detoxifying enzyme levels or defects in DNA repair. Several research groups have begun to identify genes associated with heritable forms of prostate cancer and genes, in which somatic mutations or other somatic alterations may set the stage for the development and/or progression of the disease. To this end, it has been shown by several groups that hypermethylation of the B-class glutathione S-transferase gene (GSTP1) promoter region inhibits transcription of the gene and is associated with prostate cancer development. The function of GSTP1 has been proposed as a gene that defends genomic DNA in prostate cells from environmental or endogenous DNA-damaging agents. Environmental carcinogens such as heterocyclic amines and polycyclic aromatic hydrocarbons that result from cooking meat at high temperatures may play a role as it has been shown that 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine can induce prostate cancer in rats. Reactive oxygen species (ROS), most notably the hydroxyl radical, generated endogenously by cellular metabolism are known to cause oxidative DNA damage that has been implicated in prostate carcinogenesis. Research on the development of prostate cancer suggests that symptomatic and asymptomatic chronic and acute inflammation occurs in the prostate over the life span and acts in synergy with environmental, genetic, and dietary factors to cause injury to prostatic epithelium. In response to this injury, cellular proliferation has been shown to occur. This proliferation is accompanied by oxidative stress that is related to the ongoing inflammatory process that in turn may result in high rates of oxidative damage to DNA. Other findings that implicate a role for oxidative DNA damage in prostate carcinogenesis include work by Bostwick et al. showing that SOD1, SOD2 and catalase levels are lower in prostate intraepithelial neoplasia and prostate cancer relative to benign prostate epithelium. There is also a significant increase in the proportion of mutagenic oxidatively induced DNA base lesions, 8-hydroxyadenine (8-oxoA) and 8-hydroxyguanine (8-oxoG) present in malignant prostatic tissue as well as an increase in the levels of these lesions in benign prostatic tissue with aging. Further evidence supporting the hypothesis that defective repair of oxidative DNA damage may be pivotal in prostate carcinogenesis has been provided by work on genetic polymorphisms in the base excision repair (BER) gene OGG1. Taken together these data suggest that reactive oxygen species and oxidative DNA damage may be critical in the development of prostate cancer. Using LC/MS and GC/MS, we show increased levels of oxidative DNA base damage over the baseline in PC-3 and DU-145 prostate cancer cells following exposure to ionizing radiation and a repair period. Nuclear extracts of PC-3 and DU-145 prostate cancer cell lines have defective incision of the DNA base lesions, 8-hydroxyguanine (8-oxoG), 5-hydroxycytosine (5OHC) and thymine glycol (TG) when compared to the non-malignant prostate cell line. Concomitantly, the levels of NEIL1 and NEIL2, enzymes that incise these lesions, are reduced in both cancer cell lines. Mitochondrial extracts from PC-3 and DU-145 also have defective incision of 8-oxoG compared to the control. PC-3 mitochondrial extracts are severely defective in the incision of TG and 5OHC. Consistent with the incision data, NTH1 and OGG1 2a protein levels are decreased in mitochondria of PC-3 cells. The antioxidant enzymes, glutathione peroxidase (GPx), catalase, and superoxide dismutases (SOD1, SOD2) have altered expression patterns in the cancer cell lines. Genetic analysis of the OGG1 gene reveals that both PC-3 and DU-145 cell lines harbor polymorphisms associated with a higher susceptibility to certain cancers. These data suggest that the malignant phenotype in PC-3 and DU-145 cell lines is associated with defects in base excision repair (BER), alterations in expression of BER and antioxidant enzymes, and OGG1 genetic polymorphisms.